Age-related differences in hippocampal network engagement during safety processing in adolescents.

Sound sources are typically perceived as coming from outside the listener’s head under natural listening conditions, a phenomenon known as externalization. This is a fundamental element of auditory spatial perception and also a key subjective experience for hearing an environment naturally, making its importance undeniable (). The rise of headphones and wearable devices, such as hearing aids, has further increased the significance of this research topic (). The ultimate goal of virtual reality is to create a sense of presence and enable a listener to perceive sound distance. Localizing a source outside the head is a cornerstone for achieving realism and spatial awareness (, ). However, when virtual sound sources are presented through headphones, hearing aids, or other wearable devices, the sound is often perceived as originating from inside the head, a phenomenon known as internalization. This is because the audio signal is delivered directly to the eardrum, bypassing the natural acoustic cues generated by the head and torso reflections and pinna filtering. These cues, described by Head-Related Transfer Functions (HRTFs), are critical for the subjective perception of externalization via headphones because they compensate for the lack of spatial information in the original audio signal. Additionally, when a closed-dome type is used in hearing aids or headphones, the increased low-frequency energy in the ear canal may contribute to a reduction in externalization (; ). Sound externalization is significantly related to interaural cues (). The primary interaural cues are Interaural Time Difference (ITD) and Interaural Level Difference (ILD), whose effectiveness is associated with the sound’s frequency content; high-frequency sources mainly rely on ILD, whereas low-frequency sources depend on ITD. successfully reproduced virtual sound effects that were indistinguishable from those of loudspeakers by manipulating the ITD and ILD in headphone synthesis, highlighting the central role of interaural cues in achieving externalization. However, natural sound sources have different spectral details and interaural cues at various angles. Relying solely on a single binaural parameter (either the ITD or ILD) can limit the perceived sound field between the ears, making externalization difficult to achieve (; ). This is particularly true for sound sources near the median plane, leading to a lack of spatial information and increasing the listener’s tendency to perceive the sound as internalized ().